1. Field of the Invention
The present invention relates to a method for distilling (meth)acrylic acid.
2. Description of Related Art
Acrylic acid is generally produced by subjecting gas phase catalytic oxidation reaction of propylene and/or acrolein with a molecular oxygen-containing gas, contacting a resulting acrylic acid-containing gas with water to collect as an aqueous acrylic acid solution, and separating and collecting acrylic acid from the acrylic acid solution. In the acrylic acid-containing gas, there are contained by-products such as acetic acid, formic acid, acetaldehyde, formaldehyde or the like. Among them, acetic acid is contained in relatively higher amount. In order to produce acrylic acid in a high purity, it is necessary to remove acetic acid therefrom. When acetic acid therein is intended to remove by means of distillation, the distillation temperature become higher (Boiling point of acetic acid: 118.1xc2x0 C.), thereby acrylic acid is liable to polymerize. Since acrylic acid and acetic acid have small relative volatilities, there is the problem that it is difficult to remove acetic acid from the acrylic acid solution by simple distillation.
Then, in the above collecting step, the aqueous acrylic acid solution is fed to an azeotropic separation column to be distilled (azeotropic dehydration method) in order to separate and collect acrylic acid in a high purity from the aqueous acrylic acid solution, i.e., to collect a high purity of acrylic acid substantially free from acetic acid and water by separating acrylic acid from the acetic acid and water. In the azeotropic separation column, distillation is performed in the presence of azeotropic solvents, an azeotropic mixture of acetic acid, water and the azeotropic solvent distilled from the top thereof, and acrylic acid obtained from the bottom thereof. Impurities having a low boiling point, other than acetic acid and water, can be readily removed from the solution because of their low boiling points. Thus, such impurities do not need azeotropic distillation.
However, in the above azeotropic separation column, the acrylic acid is liable to polymerize since many ingredients are present in the solution. The produced acrylic acid polymer accumulates in the column, therefore it is difficult for the azeotropic distillation column to be operated for a long period of time.
There are proposed lots of polymerization inhibitors so as to prevent the polymerization of acrylic acid. For example, U.S. Pat. No. 4,021,310 describes a polymerization inhibitor of at least one selected from the group consisting of hydroquinone, methoquinone (p-methoxyphenol), cresol, phenol, t-butylcatechol, diphenylamine, phenothiazine, and methylene blue; at least one selected from the group consisting of copper dimethyldithiocarbamate, copper diethyldithiocarbamate, and copper dibutyldithiocarbamate; and a molecular oxygen.
As a result of our research, we have found that the polymerization of acrylic acid cannot be fully prevented even if such polymerization inhibitor has been adopted. When the inhibitor is used in a prescribed amount, it does not provide full prevention effects, and there are occurred popcorn polymer and viscous polymer during distillation, thereby the continuous operation of acrylic acid production equipments including the azeotropic distillation column become impossible. Large amount of the inhibitor may be used so as to fully prevent the polymerization, but the use of the large amounts leads economical disadvantage as well as there occurred the problems of apparatus corrosion and waste water treatment. As a result, its practical use could be difficult.
The problem of polymerization in the azeotropic separation column as mentioned above is also found in the method for producing methacrylic acid by subjecting gas phase catalytic oxidation reaction of at least one selected from the group consisting of isobutylene, t-butyl alcohol and methacrolein with a molecular oxygen-containing gas.
Azeotropic separation procedure will be further explained. U.S. Pat. No. 5,315,037 describes a method for collecting a purified acrylic acid employing a distillation column as the azeotropic column in which an aqueous acrylic acid solution is azeotropically performed to distill a mixture consisting essentially of acetic acid, water, and the azeotropic solvent from the top thereof, and to collect a purified acrylic acid substantially free from acetic acid, water, and the azeotropic solvent from the bottom thereof. In addition, JP-A-10-120,618 describes a method for producing a purified acrylic acid using two distillation columns, such as an azeotropic dehydration and acetic acid separation columns, in which the bottom liquid of the azeotropic dehydration column is led to the acetic acid separation column wherein distillation is performed again to remove acetic acid. In this method, the bottom liquid of azeotropic dehydration column is cooled and then led to such an acetic acid separation column so as to heighten the separation efficiency of the acetic acid therein. However, this method relates to techniques for purifications of solutions containing acetic acid, acrylic acid, and azeotropic solvents, but does not describe prevention of polymerization in the case of distillation of (meth)acrylic acid solutions substantially free from azeotropic solvents. In addition, the solution includes the azeotropic agents, so that the concentration of (meth)acrylic acid therein becomes lowered. The method does not describe prevention of polymerization in the solutions containing higher concentration of (meth)acrylic acid.
Effects of prevention of polymerization have not been fully solved by the above methods, and it is desired to distill the above-mentioned solution of polymerizable (meth)acrylic acid for a long period of time.
In addition to the azeotropic distillation column, there can be cited, as columns wherein polymerization is liable, an aldehyde distillation column in which the aldehyde treating agent is added to the (meth)acrylic acid solution, and then the resultant is distilled to obtain a purified acrylic acid; distillation column for separating materials having high boiling points in which a purified acrylic acid is obtained from the top thereof by removing impurities having high boiling points from the (meth)acrylic acid solution or the like. The aldehyde treating agent such as hydrazine hydrate is used in the aldehyde distillation column, and acrylic acid is liable to polymerize in the distillation columns in the presence of the aldehyde treating agent and a product of aldehyde and the aldehyde treating agent.
JP-A-8-3,099 describes a method for preventing the polymerization of (meth)acrylic acid during distillation in the presence of an aldehyde treating agent using a composition for preventing polymerization comprising a p-phenylenediamine compound, phenol compound, and phenothiazine.
In the distillation column for separating materials having high boiling points, (meth)acrylic acid is liable to polymerize since the acid is exposed to high temperatures.
An object of the present invention is to provide a method for distilling a raw material liquid containing (meth)acrylic acid substantially free from azeotropic solvents for effectively preventing the polymerization of (meth)acrylic acid during distillation and for performing distillation for a long period of time in the production of (meth)acrylic acid.
As a result of our research, we have found that
(i) A (meth)acrylic acid solution from a collection column for (meth)acrylic acid is generally exposed to air for the time being, then fed to a distillation column or tower set at a prescribed temperature. A temperature difference is present between (meth)acrylic acid solution as the raw material and the entrance place, at which the raw material liquid enters, in the distillation column. When the temperature difference is high, there is occurred partial condensation or drift flow of the (meth)acrylic acid solution in the column, and polymerization is liable to occur.
(ii) The temperature of (meth)acrylic acid solution from the collection column for (meth)acrylic acid is generally dispersed. When the (meth)acrylic acid solution is stored in an intermediary tank for the time being and then fed to the column, the temperature thereof is also dispersed if the amount of piling liquids is not fixed. In such a case, polymerization of (meth)acrylic acid is liable to occur in distillation columns such as a distillation column for separating (meth)acrylic acid and the collecting solvent, aldehyde distillation column, and distillation column for separating high boiling materials.
We have found that in performing distillation, polymerization in the above column can be prevented and distillation can be stably performed for a long period of time by feeding the raw materials containing (meth)acrylic acid substantially free from azeotropic solvents which temperature is substantially equal to that of the entrance place in the distillation column to the distillation column, and finally completed the present invention.
The present invention relates to a method for distilling a raw material liquid containing (meth)acrylic acid substantially free from azeotropic solvents, collected with a collection agent from a mixed gas obtained by gas phase catalytic oxidation reactions characterized by feeding to a distillation column the raw material liquid which temperature is substantially equal to that of the entrance place in the column.
According to the present method, it can effectively prevent polymerization in each distillation column in the case of recovering (meth)acrylic acid from the (meth)acrylic acid solution substantially free from azeotropic solvents. With this procedure, a production device including the distillation column can be stably operated for a long period of time.
The above and other objects, features and advantages of the present invention will become clear from the following description of the preferred embodiments.